Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple. Besides, one NB is responsible for controlling one or more cells, and thus a user equipment (UE) establishing connection with the network means the UE establishing connection with one cell of NB.
In LTE system, Radio Resource Control (RRC) layer of the UE merely includes two RRC states, i.e. RRC_IDLE and RRC_CONNECTED; and Radio Bearers for transmitting RRC signals, generally called Signalling Radio Bearers (SRBs), are divided into the following three types: 1. SRB0 is for exchanging RRC messages over a Common Control Channel (CCCH). Uplink (UL) and Downlink (DL) both use a Transparent Mode (TM) RLC entity; 2. SRB1 is for exchanging RRC messages and upper layer Non-Access Stratum (NAS) messages over a Dedicated Control Channel (DCCH). The UL and DL both use an Acknowledged Mode (AM) RLC entity; and 3. SRB2 is for exchanging upper layer NAS messages over DCCH. The UL and DL both use the AM RLC entity.
Through use of the SRBs, the RRC layers of the UE and the network can exchange RRC messages, as a basis for radio resource settings, for completing various RRC control procedures. For example, the UE in RRC_IDLE state can establish RRC connection with the network through a RRC Connection Establishment procedure, while the UE in RRC_CONNECTED state can re-establish RRC connection with the network through a RRC Connection Re-establishment procedure.
For the above two RRC connection procedures, the RRC layer of the UE shall generate a Connection Request message (or a Connection Re-establishment Request message), and the UE shall initiate a Random Access procedure in lower layer, i.e. MAC layer, for transmitting the Connection Request message (or the Connection Re-establishment Request message) to the network over CCCH. Meanwhile, the UE can perform radio measurement for neighboring cells, and reselect to another cell when measuring results reach to cell reselection criteria, i.e. a cell re-selection occurs. Detailed operation of the above RRC connection procedures is referable in related specification and not narrated herein.
According to the current specification, when the cell reselection occurs during an ongoing RRC Connection procedure, the RRC layer of the UE would then generate a new Connection Request message (or a new Connection Re-establishment Request message) to the lower layer for transmission to a new selected cell. However, since the MAC layer is not reset and the RLC layer mapping to the SRB is not re-established as well, the Random Access procedure for an original cell is still executed, causing the Random Access procedure for the new cell cannot be correctly performed. In addition, if the Connection Request message (or the Connection Re-establishment Request message) for the original cell cannot be sent out before the cell reselection occurs, there would be two or more Connection Request messages (or Connection Re-establishment Request messages) in a transmission buffer of the RLC layer. As a result, the follow-up RRC Connection procedure cannot be completed correctly, and may cause errors, such as sending a redundant Connection Request message (or Connection Re-establishment Request message) to the new cell.